Compact digital still cameras (DSCs) and camera cell phones often utilize relatively small image sensors with minute pixel sizes to reduce the size and cost of the image sensor. However, the light gathering ability of such small image sensors is often not suitable in low ambient light conditions for the desired quality of the image captured. This is especially the case with camera cell phones because the users of such devices often take snap shots in dimly lit indoor settings. Therefore, compact digital still cameras and camera cell phones typically incorporate flashlamps which enable acceptable pictures to be taken in relatively low ambient light conditions.
A type of flashlamp commonly used in compact digital still cameras and camera cell phones is the xenon arc discharge lamp. The atoms or molecules of gas inside a glass, quartz, or translucent ceramic tube, are ionized by an electric current through the gas or a radio frequency (RF) or microwave field in proximity to the tube. The ionization results in the generation of light—usually either visible or ultraviolet (UV), although some infrared (IR) light may be emitted as well. The color temperature of the light that is emitted by an arc discharge lamp depends on both the mixture of gases or other materials inside the tube or envelope as well as the pressure and the amount and type of energization. Xenon arc discharge lamps are mostly filled with xenon gas and usually reach their peak output immediately after ignition, making them suitable for use as flashlamps in cameras.
When used in a camera device, a xenon arc discharge lamp requires a secondary stored energy source for operation, which is typically a capacitor that is charged through a circuit connected to a rechargeable battery. The capacitor is often larger than the flashlamp and this presents a problem in designing compact camera devices.
A xenon arc discharge lamp converts electrical energy into optical energy in a relatively efficient manner. However, the optical efficacy is relatively low because the emitted spectrum resembles that emitted by a black body radiator with a very high color temperature, i.e. approximately 12,000 degrees Kelvin (K). Hence, many of the generated photons have energy frequencies higher than that of visible light, i.e. they are emitted in the ultraviolet (UV) range between about 200 and 400 nanometers (nm). For efficient discharge conditions, the amount of UV radiation emitted by a xenon arc discharge lamp can actually exceed the amount of visible radiation that is emitted. For visible application like photo flash, the current density at discharge is typically decreased, trading off electrical-to-optical conversion efficiency and output of visible light. Alternatively, at high conversion efficiencies, the UV light is usually absorbed by the glass envelope of the xenon arc discharge lamp. In addition, a yellow filter is sometimes employed to reduce the amount of generated deep blue light and to adjust the color temperature of the flashlamp. FIG. 1 is a graph illustrating typical spectral distributions and window transmissions of a conventional xenon arc discharge lamp. In FIG. 1 the wavelength is in nanometers (nm) and the light output distribution is in percentages (%). In this example, the optical distribution is greater than thirty-five percent (35%) of UV light and about twenty-six percent (26%) for the visible fraction (approximately 400 nm to 700 nm).